The present invention relates to furnaces and particularly to apparatus comprising a removable damper for an air port of a chemical recovery furnace.
Wood pulp for paper making is usually manufactured according to the sulfate process wherein wood chips are treated with a cooking liquor including sodium sulfide and sodium hydroxide. The wood chips and the cooking liquor, called "white liquor", are cooked in a digester under predetermined heat and temperature conditions. After cooking, the used liquor, termed "black liquor", containing spent cooking chemicals and soluble residue from the cook, is washed out of the pulp and treated in a recovery unit where the cooking chemicals are reclaimed. Without reclamation and reuse of the cooking chemicals, the cost of the paper making process would be prohibitive.
In the recovery process, the black liquor is first concentrated by evaporation to a water solution containing about sixty-five percent solids, which solution is then sprayed into the firebox of a black liquor recovery boiler, a type of chemical reduction furnace. The chemical reduction furnace is a reactor wherein the processes of evaporation, gasification, pyrolysis, oxidation and reduction all occur interdependently during recovery of the cooking chemicals. The organic materials in the black liquor, lignin and other wood extracts, maintain combustion in the firebox, and the heat produced melts the spent cooking chemicals. A molten smelt flows out of the furnace through a smelt spout to a collection tank. Concurrently, combustion heat is employed to generate steam in a wall of boiler tubes for use as process steam and for generating electricity.
The combustion process requires the introduction of large volumes of air into the firebox, air comprising about eighty percent of the material entering the furnace. The air is forced into the firebox from windboxes or ducts disposed at several levels in surrounding relation to the firebox, through a plurality of air ports in the walls of the furnace. While variations are possible, the principal air ports are usually primary, secondary and tertiary air ports.
The primary air ports are always the smallest as well as the most numerous and are disposed on the four walls of the firebox near the bottom of the furnace and close to the char bed. The air supplied to the primary air ports is usually at a comparatively low pressure, providing a portion of the air for char bed combustion. This air is used to control the shape and position of the perimeter of the char bed. Secondary air ports, which are generally larger and fewer in number than the primary air ports, are usually disposed around the walls of the firebox higher than the primary air ports and below the level of the liquor spray nozzles. Air supplied through the secondary air ports is at a higher pressure than the primary air and is used to control the position of the top of the char bed as well as promote burning of combustible gasses rising from the char bed. Typically sixty-five to eighty percent of the total combustion air to the recovery boiler is introduced below the level of the liquor spray nozzles. The tertiary air ports are located above the liquor spray nozzles and are generally longer and fewer in number than the secondary air ports. Air supplied through the tertiary air ports is ordinarily at a still higher pressure to promote combustion and final mixing of gasses rising through the firebox.
The black liquor sprayed into the firebox, having a consistency similar to that of warm sixty weight oil, swirls, burns and falls toward the bottom of the firebox as combustion products comprising char material and smelt. The smelt and char material contact the outer walls of the firebox and, cooled by the inflowing air, form excrescent deposits around the edges of the air ports, particularly along the edges of the openings where the excrescent material builds up under influence of air rushing through the air port. Such build-ups of char material can block air flow through the ports and must be removed.
The volume and distribution of combustion air supplied to the furnace is, however, varied depending on many factors including the load of the furnace and properties of the liquor being reduced. The distribution and volume of air entering the furnace are desirably adjusted by regulating means such as dampers provided in supply ducts to the windboxes, at various locations in the windboxes, and at individual air ports for maintaining the desired air supply in all parts of the furnace. Of these three locations, the provision of regulated dampers at the air ports is most deisrable. Providing dampers at individual air ports enables the independent adjustment of mass air flow and air pressure. This independence is key because mass flow is primarily determined by smelt bed conditions, furnace geometry and air-fuel mixing needs and is nearly independent of load. The mass air flow can be controlled by controlling the relative size of the port by adjusting the damper position, while air pressure can be adjusted at a supply fan and by means of dampers within supply ducts. As the damper is closed, the aspect ratio for the air port, which is ordinarily elongated, can be made to approach equal width and height dimensions for more closely simulating a round jet of air. Such a jet is advantageous at the primary air port level as well as at secondary and tertiary air port levels because it is most energy efficient which optimizes combustion control. A more efficient jet also provides better control of the smelt bed and maintains a cleaner windbox inasmuch as cleanliness of the primary windbox is primarily affected by the proximity of the smelt bed and smelt intrusion into the windbox cavity. Maintaining a higher air pressure also helps sweep the bottom of the windbox and pushes the smelt away. Ability to control the air jet from the individual air ports and operating at higher windbox pressures further enables the operator to correct for disturbances in the char bed and otherwise correct the combustion process.
An advantageous damper construction is of the sliding or guillotine type which facilitates the control of the air port aspect ratio in the manner above mentioned whereby a comparatively high pressure jet of air can be produced. Conventional guillotine dampers operate with a pivot point located inside the windbox and slide in a track proximate the air port, the operating mechanism for the damper being contained within the windbox. The air port area is subject to smelt intrusion, thermal expansion, and warping, as well as long periods without use, causing the damper mechanisms to become frozen in a particular position particularly at the primary air port level. Removal or servicing of the damper can be difficult or impossible without closing down the furnace.